1. Field of the Invention
The present invention relates to circuit testing, and more particularly to the testing of circuits using electro-optical means.
2. Description of Related Art
Circuit testing is performed automatically by apparatus which makes contact with the circuit nodes, applies excitation signals to these nodes, and monitors the response signals produced by the tested circuit. Such apparatus is often called in the art "Automatic Test Equipment" (abbreviated as ATE).
ATE may be classified into two types, namely functional testers and in-circuit testers. In a functional tester, inputs are applied and outputs received only at the normal input and output nodes of the circuit, such as for example an edge connector for a board mounted circuit. Input signals are applied and output signals are monitored to assess the functionality of the circuit assembly as a whole.
A disadvantage of functional testers, particularly where complex circuits are involved, is that a large number of input/output combinations (referred to in the art as test patterns) are required to fully exercise the circuit, and an exhaustive test may be very lengthy. A further problem is that it may be impossible to test some circuit components, notably digital memories and counters, since certain states of such devices cannot be controlled from externally accessible nodes alone. For example, there may be no external connection to the reset input of a counter. Equally, some output conditions of some circuits may not be externally distinguishable. For these reasons, some circuit assemblies can be only partially tested with a functional tester.
On the other hand, in-circuit testers have the advantage of being able to test each component of a board mounted circuit individually, since they are designed to contact internal circuit nodes, that is nodes other than the external input and output nodes. However this advantage is obtained at the expense of a special piece of hardware, referred to in the art as a "bed of nails" fixture, comprising a plurality of spring loaded probes, positioned individually to contact nodes within a circuit. By this means component inputs and outputs may be driven and measured to establish the functionality of each component individually.
One disadvantage of in-circuit testers is the hardware required in providing a fixture adapted to each circuit to be tested, for example each board, and means for holding the board in place and in contact with the fixture. A less apparent disadvantage is possible damage to circuit devices when an output of a device is driven via a nail to a state other than that which would logically result from the signals at its inputs in order to test another device having an input connected to that output. Such backdriving, as such a circumstance is referred to in the art, can damage the backdriven device by, for example, causing excessive heating therein. Further, the miniaturization of circuits increases the difficulty of accessing the internal nodes of the circuit under test. Moreover, in-circuit testers are intrinsically poorly adapted to monitor. the overall functioning of a circuit, notably those which interfere with the respective timing of signals produced by the various components.
It is nevertheless desirable that automatic circuit testing be able to provide contemporaneously analytic information allowing validation of the functionality of the components taken individually, and further derived information allowing validation of the global functionality of the circuit, which depends not only on the functionality of each component, but also on the interactions between components. Generally, in-circuit testers give priority to the first aspect to the detriment of the second, while functional testers do essentially the opposite.
Functional testing may include a "diagnostic phase," a delicate operation for isolating the probable cause of a fault. It is generally impossible to locate some faults by exercising external inputs and monitoring external outputs alone, since there will not be a unique test pattern corresponding to every possible fault. This problem is to some extent overcome in some functional testers by providing a manual probe for use during the diagnostic phase on boards which have failed the functional test. During the diagnostic phase, an operator places the probe at various nodes of the circuit in such a way that an internal node may be monitored in a way somewhat similar to that in which it would be monitored by an in-circuit tester. Such a procedure is slow and necessitates a skilled operator, however.
The use of electro-optical materials for non-contact monitoring of electrical signals has been proposed for integrated circuits in U.S. Pat. No. 4,618,819, which describes a system which uses electro-optical crystals placed physically proximate the surface of a not yet encapsulated integrated circuit. A polarized light beam is directed towards a region of the crystal in the neighborhood of a conductor carrying an electrical signal to be measured, and then reflected. In accordance with well known laws, a characteristic of the reflected light is affected by the electric field around the conductor, such that by appropriate detection of the reflected light, the electric field and the signal which produces it may be detected. Thus an image of the electrical signal in the conductor may be obtained.
Unfortunately, electro-optical systems have not been proposed that are suitable for use in testing circuits other than integrated circuits, such as, for example, hybrid circuits and circuits assembled on printed circuit boards.